1. Field of the Invention
The present invention relates to a method for producing a magnetostrictive material which changes in length when subjected to an external magnetic field, in particular a method for producing a material of excellent magnetostrictive characteristics by controlling oxygen content of the starting material powder to be sintered into a magnetostrictive material.
2. Description of the Related Art
Magnetostriction is a phenomenon of a ferromagnetic material to undergo a dimensional change when magnetized, and a magnetostrictive material is a material which exhibits this phenomenon. Saturation magnetostrictive constant, which represents a dimensional change at saturation by magnetostriction, is generally in a range from 10−5 to 10−6, and a magnetostrictive material having a high saturation magnetostrictive constant is sometimes referred to as a supermagnetostrictive material. These materials are widely used for vibrators, filters, sensors, and the like. At present, a magnetostrictive material based on a laves type intermetallic compound of RFe2 which is a compound of R (rare-earth element) and Fe is known to have a high saturation magnetostrictive constant (refer to U.S. Pat. Nos. 3,949,351, 4,152,178, 4,308,474 and 4,375,372). These materials, however, involve problems of insufficient magnetostrictive value in an external magnetic field of low intensity, although exhibiting a high value when applied to a field of high intensity. Therefore, magnetostrictive materials based on a laves type intermetallic compound of RFe2 have been studied to have a higher magnetostrictive value even in an external magnetic field of low intensity, and it is proposed to orient the material along the [111] axis as an easy-magnetization axis of high magnetostrictive constant. Magnetostrictive materials based on a laves type intermetallic compound of RFe2 exhibit a high magnetostrictive value at a composition of Tb0.3Dy0.7Fe2.0 (atomic ratio), and this composition has been used exclusively.
JP-A 7-286249 proposes a method for producing a magnetostrictive material, where a mixture of three types of alloy powders, A: composed of Tb, Dy and T (iron group element) B: composed of Dy, Tb and T, and C: composed of T, is sintered to secure a high degree of orientation when compacted in a magnetic field. This method, however, involves problems such that density of the sintered magnetostrictive material being not always sufficient.
In order to solve these problems, JP-A 2002-129274 proposes a method for producing a magnetostrictive material by sintering a mixture composed of Starting Materials A, B and C, where Starting Material A is represented by Formula 1 (TbxDy1-x)Ty (T is at least one metallic element selected from the group consisting of Fe, Ni and Co, 0.35<x≦0.50 and 1.70≦y≦2.00), Starting Material B is represented by Formula 2 DytT1-t (Dy may contain Tb and/or Ho, and 0.37≦t ≦1.00) and contains hydrogen at 7000 to 22000 ppm, inclusive, and C contains T, to produce a magnetostrictive material represented by Formula 3 (TbvDy1-v)Tw (0.27≦v<0.50, and 1.70≦w≦2.00).
JP-A 2002-129274 discusses that Starting Material B, when hydrogen is absorbed therein, has the particles cracked by internal stresses resulting from the strains which the hydrogen atoms cause either by forming a hydride or penetrating into the crystal structure. When a mixture of Starting Materials A, B and C is formed into a compact under pressure, the stresses are concentrated at the cracked edges to further propagate the cracks, with the result that the compact is sintered more densely by virtue of the finely divided particles penetrating into interspaces between the Starting Material A particles.
The method for producing a magnetostrictive material proposed by Japanese Patent Laid-Open No. 2002-129274, however, involves releasability-related problems, because it needs a high pressure for releasing a compact from a die (hereinafter referred to as releasing pressure). Use of a lubricant for reducing releasing pressure is one approach to those problems. This, however, tends to deteriorate the magnetostrictive characteristics of the product, because it can allow carbon to remain massively in the form of carbide phase in the sintered body, which inhibits elongation of the magnetostrictive material.